This invention pertains to the art of fuel burning devices and, more particularly, to a fuel burning device including an oxidation catalyst.
The invention is particularly applicable to woodburning stoves. The invention can be readily used in other fuel burning appliances in which an oxidation catalyst is employed, particularly in other solid fuel appliances.
It is highly desirable to control the temperature at which combustion occurs in the stove. When combustion occurs within the optimum range for heating a room to a desired temperature, the inhabitants of the room will be as comfortable as possible; clearly this will not be the case if the combustion occurs at a higher or a lower temperature or if its temperature oscillates widely. If the combustion temperature is too high, in addition to causing discomfort, the stove will consume more fuel than is necessary. In addition, with the stove at a high temperature, there is a danger of chimney fire due to high stack temperatures which are produced. Finally, an unnecessarily hot stove increases the danger of a person in the room suffering a severe burn from accidentally touching it or the stove igniting something flammable which is near the stove.
If the stove combustion temperature is too low, on the other hand, efficiency is again reduced, excessive CO and creosote is formed and there is a danger that the fire will go out, presenting the inconvenience of having to restart it.
Woodburning stoves equipped with thermostats which attempt to maintain the combustion temperature at the desired level are known. Such a stove is disclosed in U.S. Pat. No. 4,117,824, issued Oct. 3, 1978, to McIntire et al. In the McIntire et al patent, a damper is mounted in an air intake and can be rotated about its edge to control the quantity of air supplied to the stove combustion chamber. This controls the combustion temperature. A bimetallic coil is provided outside the air intake duct. One end of the coil is connected to the damper, and the other end of the coil is connected to a control knob. When the coil heats as a result of a rise in the stove temperature, it rotates and moves the damper in the closed direction. Conversely, a drop in the stove temperature as measured by the bimetallic coil results in the damper being opened further. The position of the damper can also be controlled manually by turning the control knob.
The combustion temperature in wood burning stoves with known thermostats typically exhibits oscillations of a very large amplitude, often reaching 70.degree.-300.degree. F. This is because known thermostats typically are not sufficiently sensitive to changes in the stove temperature. Such thermostats generally respond to temperature changes only very slowly and display a response that is very small in magnitude. As a result, by the time the thermostat reduces the air supply responsive to a temperature rise, the temperature has risen very high, and the amount of the reduction in the air supply is correspondingly large. Because of this, the combustion temperature falls dramatically. Due to its relative insensitivity, however, the thermostat detects the temperature drop only after the temperature has fallen as much as 70.degree.-300.degree. F. below its peak. The thermostat responds by drastically increasing the air supply, allowing the combustion temperature to rise very quickly to a new peak. As a result, the stove temperature oscillates wildly, instead of remaining at a stable level.
A stove controlled by such a thermostat thus reaches temperatures both substantially higher and substantially lower than the optimum combustion temperature and accordingly presents many of the risks and suffers from many of the drawbacks of manually controlled stoves.
Conventional woodburning stoves, fireboxes or fireplaces do not burn all the combustible substances of a conventional fuel such as wood. The smoke and gas effluent of a wood fire normally contains creosotes and substantial quantities of oxidizable substances such as combustible gases. Such gases can condense and become attached to a flue passageway during the emission of the effluent to the environment. Continued condensation and attachment may result in a particularly undesirable fire hazard in a flue or chimney, substantially hampering the efficiency of the burning device, and polluting the atmosphere.
In a wood burning operation, at a temperature of 250.degree. F., oxidizable effluent gases are completely fogged (condensed droplets) while at 450.degree. F. the effluent is 70-80% gas with the remainder comprising condensed droplets. Since the condensed droplets will not oxidize in a catalyst, an effluent reheating method or element has been necessary to raise the temperature of the effluent such that the condensed droplets would again become gaseous. Alternatively, the effluent was kept extremely hot, often by overfiring the stove.
Oxidation catalysts have been employed in combination with other fuel burning or incinerator devices for combusting smoke, creosotic flue gases and other objectionable components in the effluent. In order to promote such combustion, some prior art devices have employed various methods to reheat creosotic gases which have condensed to droplets during travel from the burning fuel to the catalyst.
Generally, however, none of the apparatus found in the prior art provided a system by which the combustion gases and secondary air could be homogeneously mixed before delivery to the catalyst nor did previous systems take advantage of the heat energy radiated from the catalyst to achieve a combustion function in the mixture of gases even before they entered the catalyst.
Additionally, most systems failed to take advantage of the heat emanating from the firebox itself, as opposed to providing a secondary heat source, to preheat the secondary air.
Also, the delivery rate of the secondary air has been inadequately controlled and has not been uniform. Moreover, insufficient attention has been paid to preventing the secondary air flow from becoming restricted. These failures have contributed to the inefficiency found in systems which comprise the prior art.
In other catalytic woodburning stoves found in the prior art, it has been necessary to operate the stove at a temperature higher than was desirable for residential operation, in order to operate the catalyst device without reheating the effluent. Such stoves would often consume six or more pounds of wood per hour in order to prevent some of the effluent from cooling to a temperature near 250.degree. F. and condensing as it passed from the woodburning flame to the catalyst. Substantial eddying of effluent along the stove top and side walls would cause the effluent to cool. To prevent such cooling an undesirably high temperature had to be maintained so that the effluent would contain predominantly gases as opposed to condensed creosote droplets.
The present invention overcomes the above referred to problems. It provides a new catalytic stove which is simple in design, economical to manufacture and adaptable for use in residential environment. It is easy to install, and it operates at a temperature which is not undesirably hot, unsafe, or wasteful of energy. The present invention provides improved catalytic oxidation of effluent from a burning fuel.
The present invention combines a controlled air delivery system, a unique catalytic dome with a uniquely controlled and directed air flow. It typically lowers fuel useage from an objectionable six or more pounds per hour (a burning rate higher than normally encountered in homes) to a heretofore unobtainable two pounds per hour (a rate commonly desired in homes). This economy greatly expands the operating range of a catalytic woodburning stove.